1. Field of the Invention
The invention concerns an air bearing and a use of such an air bearing.
2. Description of the Prior Art
In contrast to conventional bearings, known air bearings are characterized by their practically negligible friction and aging as well as by their high-precision and high-speed capability. The absence of oil is likewise of decisive importance because this significantly expands the field of use (for example in dusty environments), and the maintenance of the oil system is simultaneously foregone. Instead of the oil, in an air bearing an air film or an air flow in a relatively narrow air column produces a contact-free bearing. A stator and a rotor to which a body is attached interact via the air film.
The basic positive properties of air bearings have facilitated the entry of air bearings into the medical technology field. The requirements for an air bearing in this field can be summarized as being that they should run without need of maintenance, quietly and stably, over a wide temperature range, independent of the rotational frequency. Optimally small installation volumes and cost-effective solutions are additionally required.
An example of an air bearing that approaches these properties required in medical technology is commercially available from the company NEWWAY with headquarters at 50 McDonald Blvd., Aston, Pa., 19014, USA, under the name Air Bearing Slewing Rings, and is depicted in a cross-section representation in FIG. 1. This type of an air bearing is in the form of a positive air bearing 1 in which, according to one embodiment, a stator 2 of annular design, which can be mounted firmly in an apparatus, has on its inner side, a rotor 3 that is fashioned in one piece and likewise in annular form. The rotor 3 is completely enclosed by the stator 2 at its outer side in the radial direction (characterized by the first arrow 4) and at least partially in the axial direction (characterized by the second arrow 5). Conforming to an external, circumferential (peripheral) rotor surface that forms a radial rotor bearing surface, the stator 2 has a circumferential radial stator bearing surface 7 that is fashioned for supportive bearing of the rotor 3 or of a body connected with the rotor 3 in the radial direction 4 of the rotor 3. The stator 2 additionally has circumferential axial stator bearing surfaces 9 (corresponding to the body surfaces of the rotor 3 that are oriented in the axial direction 5 and form axial rotor bearing surfaces 8) that are fashioned for supportive bearing of the rotor 3 or of a body connected with the rotor in the axial direction 5. To generate the air gap necessary for bearing, the radius of the radial rotor bearing surface 6 and the radius of the radial stator bearing surface 7 differ slightly. The air gap formed upon operation has an essentially homogeneous thickness along the bearing surfaces 7 in the circumferential direction.
This type of positive air bearing 1 does in fact satisfy the criterion of the space-saving design. This known air bearing 1, however, exhibits relatively tight limit values with regard to its operating temperatures, which hinders the use of this air bearing. This problem occurs because—due to the 360° design of the stator 2 surrounding the rotor 3—the air bearing 1 may be operated only with a slight temperature difference between the rotor 3 and the stator 2, since otherwise the danger exists that excessive heating of the rotor 3, and an associated expansion of the rotor 3, can lead to a seizing of the rotor 3 and the stator 2. This situation is exacerbated by the use of different materials. The rotor 3 is typically made of steel and the radial stator bearing surface is made of a porous graphite. Since steel has a much greater value of the coefficient of thermal expansion than graphite, in this known bearing seizing between stator and the positively embedded rotor is inevitably to be expected. Although this could in principle be counteracted by two measures, neither of these measures nor their consequences are desirable. One possibility would be to continuously cool the rotor 3, but this is cost-intensive and complicated. Another possibility would be to increase the air gap thickness between the rotor 3 and the stator 2, but the effectiveness of the air bearing would then be decreased, because the thickness of the air gap must be relatively small (for example in the range of 1/1000 mm) in order to obtain an effective bearing. It should also be noted that, very generally, an air bearing 1 with a smaller air gap can bear (support) a larger load than an air bearing 1 with a larger air gap.